The emulator receives keyboard input from SDL and generates a framebuffer which it passes back to SDL. SDL is only used to handle the grunt work of reading keyb

The emulator receives keyboard input from SDL and generates a framebuffer which it passes back to SDL. SDL is only used to handle the grunt work of reading keyboard input and displaying the framebuffer in a window, which is not the interesting part of writing a NES emulator. It's also not a necessary part of an emulator -- it's actually easier to write an emulator that reads controller inputs from a TAS movie file and writes its output to a video file. Making it interactive is just more fun.